In a typical sand casting process, a molten material is introduced into a casting cavity through a fluid passage called a runner system. The runner system typically includes one or more risers. Risers are reservoirs of liquid metal positioned adjacent features called gates. The gates are the points at which the molten metal enters the casting cavity. The presence of ample molten material in the reservoirs militates against the formation of cavities or voids in the casting due to shrinkage of the molten material during solidification thereof. However, when the cast object is removed from the mold, the runner system, including the risers and gates, remains attached to the casting. At some point in the foundry process the casting must be separated from the runner system. This process step is called de-gating.
Typically, at some time after the cast object has been removed from the expendable sand mold, the runner system is removed using one of the following methods: 1) apply an impact force to the riser, causing the gate connection to fracture; 2) pry the casting and riser apart using a mechanically powered wedge device positioned between the casting and riser, causing the gate connection to fracture; 2) melt the gate connection using an oxy-acetylene or plasma torch; 4) fracture the gate connections using a trim press equipped with tooling specific to the casing design; or 5) sever the gate connections using a band saw, circular saw, or abrasive wheel.
A band saw or large circular saw is typically used to remove the risers from aluminum cylinder block castings. These castings typically require a gate connection at multiple locations on the cylinder head mounting face. Two or more of these gate connections are often attached to a single riser feature. De-gating methods other than sawing are not feasible due to the risk of mechanical or thermal damage to the casting or limited accessibility of the connecting points.
Sawing is preferably done at or near room temperature, requiring a cooling period of the cast object. However, it is desirable to perform a heat treatment on the cast object as soon as possible after the casting process to take advantage of the residual heat in the cast object and to minimize casting inventory. Sawing is also adversely affected by the presence of sand in the cutting region. Therefore, removal of the risers typically does not occur until after the heat treatment process which also removes much of the sand from the cutting region.
The presence of risers during the heat treatment process increases the time and energy required for heating and cooling the casting. Also, the cast object may crack during quenching due to different cooling rates of the riser and the casting. A larger heating furnace is also needed due to the increased weight and volume of the cast object including the risers. Further, removal of sand from internal passages during heat treatment is inhibited by the presence of risers, adversely affecting quenching and internal casting cleanliness. Failure to adequately remove sand from a sawed region also adversely affects saw blade life.
It would be desirable to develop a method and apparatus for the separation of a first structure at a connecting point to a second structure, wherein damage to the first structure and costs of production are minimized.